Physicists pinpoint the perfect crystal site for an ultra-accurate nuclear clock

Physicists pinpoint the perfect crystal site for an ultra-accurate nuclear clock

Growing crystals “like alchemy,” researchers plotted out the ideal site within the lattice to build a nuclear clock

Today’s best atomic clocks, which track time by measuring the way electrons consistently jump between energy states when exposed to particular laser light, would lose less than one second of accuracy over the entire age of the universe so far. They are incredibly accurate, but also incredibly delicate: an entire lab’s worth of vacuum chambers and shielding is required to prevent interference from stray electric or magnetic fields. To fix these issues scientists aim to go one layer deeper into the atom, from the electrons on the outside t o the nucleus nestled within , and track how energy flips a single neutron between quantum states. According to Thorsten Schumm, a physicist at the Vienna University of Technology, such a switch could make the clocks up to 10 times more precise, and the atom itself would shield the clock from some outside interference. Plus, such atoms could be embedded protectively in a crystal.

In a new study in Science , Schumm and his colleagues have mapped the four places a thorium atom can sit inside a crystal and f ound one close to ideal for building the most precise clock ever .

In most atoms, the energy needed to flip a neutron in this way would require a gamma-ray laser, which scientists can’t make yet. There’s just one known exception, an isotope called thorium 229, which researchers showed in 2024 can be triggered with an ultraviolet laser instead.

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“Thorium 229 is a quirk of nature. Its nuclear transition energy is remarkably low, making it accessible to high-purity laser light,” explains Andrei Derevianko, a physicist at the University of Nevada, Reno, who was not involved in the study. Researchers sought a crystal structure that could house and protect thorium 229 atoms, removing the need for intensive vacuum chambers for the clock to function.

“Crystal growing is like alchemy, or like cooking,” Schumm says. “You try, and then you try something else, and then you keep trying.” After 15 years of experiments, Schumm’s group ended up with a recipe for solid calcium fluoride crystals, doped with little impurities of thorium 229, that looked like transparent millimeter-sized cubes. “In the end it looks like a tiny little piece of glass, but there’s tens of thousands of hours of learning how to do that,” he adds. The researchers realized, however, that if they targeted thorium 229 in a bad internal position, the crystal would have an uneven electric field that would harm the clock’s timing.